Process for the stereoselective preparation of phenylisoserine derivatives used in making taxols

ABSTRACT

A stereoselective process for preparing phenylisoserine derivatives is disclosed. Benzylamine is reacted with an agent for introducing a phenyl or a t-butoxycarbonyl group. The product undergoes double anionization and then is reacted with acrolein to provide a mixture of alcohol syn and anti diasteroisomers. The syn isomer is isolated by chromatography. Whereupon, the hydroxyl is protected and the product is oxidized to provide the phenylisoserine derivative.

FIELD OF THE INVENTION

The present invention relates to a process for the stereoselectivepreparation of phenylisoserine derivatives of the general formula##STR1## in which R is a phenyl radical or a tert-butoxy radical and R₁is a protecting group for the hydroxyl group.

DESCRIPTION OF THE INVENTION

In general formula (I), R₁ is more particularly a methoxymethyl,1-ethoxyethyl, benzyloxymethyl, (β-trimethylsilylethoxy)methyl,tetrahydropyranyl or 2,2,2-trichloroethoxycarbonyl radical. The radicalR₁ is preferably the 1-ethoxyethyl radical.

The procedure of general formula (I) are useful for preparing thebaccatin III and 10-deacetylbaccatin III derivatives of the generalformula ##STR2## in which R is a phenyl radical or a tert-butoxy radicaland R₂ is a hydrogen atom or an acetyl radical.

The products of general formula (II) in which R is a phenyl radicalcorrespond to taxol and 10-deacetyltaxol and the products of generalformula (II) in which R is a tert-butoxy radical correspond to thosedescribed in European patent 253 738.

The products of general formula (II), and in particular the product ofgeneral formula (II) in which R₂ is a hydrogen atom and which is in the2'R,3'S form, have particularly valuable antitumoral and antileukaemicproperties.

The products of general formula (II) can be obtained by reacting aproduct of general formula (I) with a taxane derivative of the generalformula ##STR3## in which R₃ is an acetyl radical or a protecting groupfor the hydroxyl group and R₄ is a protecting group for the hydroxylgroup, and then replacing the protecting groups R₁ and R₄ and, ifappropriate, R₃ with a hydrogen atom under the conditions described byJ-N. DENIS et al., J. Amer. Chem. Soc., 110(17) 5917-5919 (1988).

It is possible to react the racemic product of general formula (I) andsubsequently to separate the diastereoisomers of the product of generalformula (II), or else to react each of the enantiomers of the product ofgeneral formula (I) separately with the product of general formula(III).

According to the present invention, the acid of general formula (I) (synform, racemic mixture) can be obtained from benzylamine.

By reaction with an agent for introducing a benzoyl or t-butoxycarbonylgroup, benzylamine is converted to a product of the general formula##STR4## in which R is as defined above, which, after doubleanionisation, is reacted with acrolein to give the alcohol of thegeneral formula ##STR5## in which R is as defined above, in the form ofa syn and anti mixture containing essentially the syn form: ##STR6##

The alcohol of general formula (Va), previously separated from themixture of the syn and anti forms, is oxidized to the acid of generalformula (I) after protection of the hydroxyl group.

The product of general formula (IV) is generally obtained by reactionwith an agent for introducing a benzoyl or t-butoxycarbonyl group,preferably benzoyl chloride or di-t-butyl dicarbonate, as the case maybe. The reaction is generally carried out in an organic solvent such asmethylene chloride, in the presence of an inorganic base such as sodiumhydroxide or sodium bicarbonate or carbonate, or an organic base such astriethylamine or 4-dimethylaminopyridine, at a temperature of between 0°and 50° C.

The double anionization of the product of general formula (IV) isgenerally carried out using equivalents of an organolithium derivativesuch as s-butyllithium, in an anhydrous organic solvent such astetrahydrofuran, at a temperature below -50° C. and preferably of about-78° C.

The reaction of acrolein with the dianion of the product of formula (IV)is generally carried out by adding acrolein, preferably freshlydistilled, to the solution of the dianion, previously cooled to about100° C. After hydrolysis, the product of general formula (V) is obtainedin the form of a mixture of the syn and anti diastereoisomers, fromwhich the syn form of formula (Va) is separated by chromatography.

Protection of the hydroxyl group of the alcohol of general formula (Va)is effected under the normal conditions for the preparation of ethersand acetals, for example in accordance with the processes described byJ-N. DENIS et al., J. Org. Chem., 51, 46-50 (1986).

Oxidation of the protected alcohol of general formula (Va) is preferablycarried out by means of an alkali metal periodate (sodium periodate), inthe presence of a catalytic amount of a ruthenium salt (RuCl₂) andsodium bicarbonate, in an aqueous-organic medium such as, for example, acarbon tetrachloride/acetonitrile/water mixture. The reaction isgenerally carried out at a temperature of about 20° C.

Oxidation can also be carried out by means of potassium permanganate,for example in the presence of adogen in a pentane/water mixture, or inthe presence of aliquat or dicyclohexyl-18 crown-6 in methylene chlorideor in a pyridine/water mixture. It is also possible to usetriethylbenzylammonium permanganate in the presence of pyridine inmethylene chloride.

The product of general formula (I) (syn form, racemic mixture) can beresolved into its enantiomers, and in particular into its 2R,3Senantiomer, for example in accordance with the process described by D.Petterson, Thesis at the University of Lund (Sweden), pages 27-28(1989).

EXAMPLES

The following Examples, which are given without implying a limitation,show how the invention can be put into practice.

EXAMPLE 1

218.5 μl (214.3 mg, 2 mmol) of benzylamine and 10 cm³ of dry methylenechloride are introduced under argon into a 50 cm³ single-necked flasksurmounted by a condenser and equipped with a magnetic stirring system.418 μl (303 mg, 3 mmol) of triethylamine and, in small portions(exothermic reaction), 524 mg (2.4 mmol) of pure di-t-butyl dicarbonateare added to the solution obtained. When the addition is complete, thereaction is left to proceed for 4 hours at a temperature of about 20° C.and the resulting reaction mixture is then diluted with 40 cm³ ofmethylene chloride. The organic phase is washed 4 times with 5 cm³ ofwater and once with 5 cm³ of a saturated aqueous solution of sodiumchloride. The organic phase is dried over anhydrous sodium sulphate.After filtration, the methylene chloride is driven off under reducedpressure on a rotary evaporator. The residue obtained (505 mg) ispurified by chromatography on a column of silica gel using an ethylacetate/methylene chloride mixture (5/95 by volume) as the eluent. 406mg (1.96 mmol) of t-butyl benzylcarbamate are thus obtained in the formof a white solid with a yield of 98%, said product having the followingcharacteristics:

melting point: 55.5°-56.5° C. (hexane)

infrared spectrum (film): characteristic absorption bands at 3350, 3315,3080, 3060, 3040, 3010, 2980, 2960, 2930, 1680, 1550, 1450, 1442, 1395,1370, 1315, 1290, 1255, 1180, 1140, 1080, 1055, 1035, 950, 930, 918,865, 770, 750, 725 and 700 cm⁻¹

proton nuclear magnetic resonance spectrum (300 MHz; CDCl₃ ; chemicalshifts in ppm; coupling constants J in

Hz) 1.46 (s, 9H); 4.3 (d, J=5.7, 2H); 4.84 (s broad, 1H); 7.22-7.34 (m,5H)

¹³ C nuclear magnetic resonance spectrum (CDCl₃): 28.38 (CH₃); 44.69(CH₂); 79.43 (C); 127.27 (CH); 127.41 (CH); 128.54 (CH); 138.93 (C);155.84 (C)

EXAMPLE 2

4.2 g (20.3 mmol) of t-butyl benzylcarbamate, 40 cm³ of anhydroustetrahydrofuran and 6.5 cm³ (5.0 g, 43 mmol) oftetramethylethylenediamine (TMEDA) are introduced successively into a250 cm³ single-necked flask placed under argon and equipped with amagnetic stirring system. The solution obtained is cooled to 78° C. and60 cm³ (60 mmol) of a 1M solution of secondary butyllithium in hexaneare then added dropwise. The reaction is left to proceed for 3 hours atthis temperature and the mixture is then cooled to -100° C. 3 cm³ (2.5g, 44.9 mmol) of freshly distilled acrolein are then added and thereaction is left to proceed for 3 to 4 minutes at this temperature andthen for 1 hour at -78° C. The resulting reaction mixture is hydrolyzedat -78° C. with 20 cm³ of water and then extracted with 2 times 30 cm³of ether. The organic phases are combined and then washed twice with 20cm³ of water and once with 10 cm³ of a saturated aqueous solution ofsodium chloride. They are then dried over anhydrous sodium sulphate.After filtration, the solvents are driven off under reduced pressure.The residue obtained (11.6 g) is purified on a column of silica gelusing a methylene chloride/ether mixture (95/5 by volume) as the eluent.2.606 g (9.91 mmol) of1-phenyl-1-t-butoxycarbonylamino-2-hydroxybut-3-ene are obtained with ayield of 49% in the form of a mixture of the syn and antidiastereoisomers in a ratio of 6/1.

The syn diastereoisomer is separated from the anti diastereoisomer bychromatography on a column of silica gel using an ether/hexane/methylenechloride mixture (5/45/50 by volume) as the eluent.

The syn diastereoisomer has the following characteristics:

melting point: 86.5°-88° C. (hexane)

infrared spectrum (film): characteristic absorption bands at 3400, 2975,2920, 1690, 1500, 1450, 1390, 1365, 1250, 1175, 1080, 1050, 1020, 995,920, 755 and 700 cm⁻¹

proton nuclear magnetic resonance spectrum (300 MHz; CDCl₃ ; chemicalshifts in ppm; coupling constants J in Hz): 1.40 (s, 9H); 1.9 (s broad,1H); 4.38 (pst, J=4.6 and 4.8, 1H); 4.70 (s broad, 1H); 5.20 (dt, J=1.4and 10.5, 1H); 5.26 (s broad, 1H); 5.34 (dt, J=1.4 and 17.2, 1H); 5.86(ddd, J=54, 10.5 and 17.2, 1H); 7.24-7.37 (m, 5H)

¹³ C nuclear magnetic resonance spectrum (CDCl₃) 28.12 (CH₃); 58.74(CH); 75.33 (CH); 79.58 (C); 116.36 (CH₂); 126.69 (CH); 127.26 (CH);128.32 (CH); 137.17 (CH); 139.96 (C); 155.89 (C)

mass spectrum (c.i.) (NH₃ +isobutane): 321 (M⁺ +isobutane); 281 (MH⁺+NH₃); 264 (MH⁺, parent peak); 246, 225, 208, 190, 164, 124, 106

elemental analysis:

calculated % C 68.41 H 8.04 N 5.32

measured % C 68.15 H 7.98 N 5.34

The anti diastereoisomer has the following characteristics:

infrared spectrum (film): characteristic absorption bands at 3370, 3060,2975, 2920, 1680, 1530, 1470, 1290, 1250, 1170, 1040, 1000, 930, 900,870, 840, 755 and 700 cm⁻¹

proton nuclear magnetic resonance spectrum (300 MHz; CDCl₃ ; chemicalshifts in ppm; coupling constants J in H₂): 1.41 (s, 9H); 1.8 (s broad,1H); 4.43 (psq, J=0.9 and 4.4, 1H); 4.78 (s broad, 1H); 5.18 (dt, J=1.2and 10.5, 1H); 5.24 (s broad, 1H); 5.26 (dt, J=1.2 and 17, 1H); 5.71(ddd, J=5.5, 10.5 and 17, 1H); 7.24-7.36 (m, 5H)

¹³ C nuclear magnetic resonance spectrum (CDCl₃): 28.23 (CH₃); 59.22(CH); 75.33 (CH); 79.85 (C); 117.06 (CH₂); 127.29 (CH); 127.56 (CH);128.33 (CH); 136.27 (CH); 138.14 (C); 155.61 (C)

elemental analysis:

calculated % C 68.41 H 8.04 N 5.32

measured % C 68.43 H 8.14 N 5.08

EXAMPLE 3

526 mg (2.0 mmol) of1-phenyl-1-t-butoxycarbonylamino-2-hydroxybut-3-ene, syn form, 20 cm³ ofdry methylene chloride, 1.9 cm³ (20.0 mmol) of distilled ethyl vinylether and 50.2 mg (0.2 mmol) of pyridinium p-toluenesulphonate (PPTS)are introduced successively into a 50 cm³ single-necked flask placedunder an argon atmosphere and equipped with a magnetic stirring system.The resulting homogeneous reaction mixture is left to react for 4.5hours at a temperature of about 20° C. When the reaction is complete, 1drop of pyridine is added and the reaction mixture is then diluted in 60cm³ of methylene chloride. The organic phase is washed twice with waterand twice with a saturated aqueous solution of sodium chloride and thendried over anhydrous sodium sulphate. After filtration, the solvents aredriven off under reduced pressure on a rotary evaporator. The residueobtained is purified by passage over a column of silica gel using ahexane/ether mixture (8/2 by volume) as the eluent. 580 mg (1.73 mmol)of 1-phenyl-1-t-butoxycarbonylamino-2-(1-ethoxyethoxy)but-3-ene areobtained with a yield of 87% in the form of two epimers in a ratio of55/45, said product having the following characteristics:

melting point: 66°-72° C.

infrared spectrum (film): characteristic absorption bands at 3370, 2970,2925, 2875, 1680, 1520, 1495, 1365, 1285, 1250, 1170, 1080, 1050, 1005,955, 930, 890, 870, 755 and 705 cm⁻¹

proton nuclear magnetic resonance spectrum (300 MHz; CDCl₃ ; chemicalshifts in ppm; coupling constants J in Hz): 0.9 (min) and 1.07 (maj)(2t, J=7, 3H); 1.05 (min) and 1.22 (maj) (2d, J=5.3 (min) and 5.4 (maj),3H); 1.40 (s, 9H); 2.90-2.98 and 3.05-3.51 (m, 2H); 4.16 and 4.23(2psdd, J=6.6 and 7, 1H); 4.31 (min) and 4.62 (maj) (2q, J=5.3 (min) and5.4 (maj), 1H); 4.71 (maj) and 4.73 (min) (2m, 1H); 5.22 and 5.23 (2dt,J=1.2 and 10.5, 1H); 5.25 and 5.30 (2dt, J=1.2 and 17.4, 1H); 5.37 and5.44 (2m, 1H); 5.77 (min) and 5.91 (maj) (2ddd, J=7, 10.5 and 17.4, 1H);7.17-7.37 (m, 5H)

elemental analysis:

calculated % C 68.03 H 8.71 N 4.18

measured % C 68.00 H 8.78 N 4.13

EXAMPLE 4

A solution of 251 mg (0.75 mmol) of1-phenyl-1-t-butoxycarbonylamino-2-(1-ethoxyethoxy)but-3-ene, syn form,in 1.5 cm³ of acetonitrile is introduced into a 15 cm³ single-neckedflask placed under an argon atmosphere and equipped with a magneticstirring system. 1.5 cm³ of carbon tetrachloride, 2.25 cm³ of distilledwater and, with thorough stirring, 409.5 mg (4.875 mmol) of sodiumbicarbonate are then added successively. 882 mg (4.125 mmol) of sodiumperiodate are then added in small portions. The reaction medium is leftto react for 5 minutes, with stirring (evolution of gas), and 25.1 mg(10% by weight) of RuCl₃ are then added all at once. The reactionmixture, which has turned black and become highly heterogeneous, is leftto react for 48 hours at a temperature of about 20° C., with vigorousstirring.

The reaction mixture is diluted with water to give a total volume of 12cm³. The black basic aqueous phase is extracted 3 times with 20 cm³ ofether. The basic phase is then cooled to 0° C., after which it istreated dropwise with 3 cm³ of a 2M aqueous solution of hydrochloricacid, in the presence of 30 cm³ of methylene chloride, with vigorousstirring. The resulting acidic aqueous phase is extracted 8 times with35 cm³ of methyl chloride. The organic phases are combined and washedwith 3 times 8 cm³ of water and 1 times 10 cm³ of a saturated aqueoussolution of sodium chloride. They are dried over a 1/1 (w/w) mixture ofsodium sulphate and magnesium sulphate and filtered under reducedpressure on Celite. The solvents are driven off under reduced pressureto a volume of 5 to 8 cm³. The residue is dried over a 4 molecularsieve. The organic phase is separated from the molecular sieve and theremaining solvent is then driven off on a rotary evaporator.

205 mg (0.58 mmol) of pure3-phenyl-3-t-butoxycarbonylamino-2-(1-ethoxyethoxy)propionic acid, synform, are obtained with a yield of 77% in the form of a pale yellow oilhaving the following characteristics:

infrared spectrum (film): characteristic absorption bands at 3700-2200,3060, 2980, 2930, 2850, 1720, 1660, 1602, 1590, 1500, 1450, 1400, 1370,1280, 1250, 1170, 1080, 1050, 1030, 955, 930, 890 and 700 cm⁻¹

proton nuclear magnetic resonance spectrum (300 MHz; CDCl₃ ; chemicalshifts in ppm; coupling constants J in Hz); 0.81 and 1.04 (2t, J=7, 3H);1.18 and 1.20 (2d, J=5.4, 3H); 1.42 (s, 9H); 2.60-2.88 and 3.15-3.52 (m,2H); 4.35-4.50 and 4.65-4.80 (m, 2H); 5.29 (s broad, 1H); 5.72 (s broad,1H); 7.13-7.38 (m, 5H); 8.52 (s broad, 1H).

Although the invention has been described in conjunction with specificembodiments, it is evident that many alternatives and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, the invention is intended to embrace all ofthe alternatives and variations that fall within the spirit and scope ofthe appended claims. The above references are hereby incorporated byreference.

We claim:
 1. Process for the stereoselective preparation ofphenylisoserine derivatives of formula ##STR7## in which R is a phenylor tert-butoxy radical and R₁ is a protecting group for the hydroxylgroup, comprising treating benzylamine with an agent for introducing abenzoyl or t-butoxycarbonyl group to give the product of formula##STR8## which, after double anionization, is reacted with acrolein togive the alcohol of formula ##STR9## in the form of a syn and antimixture, from which the syn form: ##STR10## is separated, the hydroxylgroup of said syn form is protected and the alcohol protected in thisway is then oxidized to give the phenylisoserine derivative of formula(I) in the syn form, which is optionally separated into enantiomers. 2.Process according to claim 1, wherein the agent for introducing thebenzoyl or t-butoxycarbonyl group is benzoyl chloride or di-t-butyldicarbonate, which is reacted with benzylamine in an inert organicsolvent, in the presence of an inorganic base, or an organic base. 3.Process according to claim 1, wherein acrolein is reacted withN-benzylbenzamide or t-butyl benzylcarbamate, previously anionized byreaction with 2 equivalents of an organolithium derivative, in an inertorganic solvent, at a temperature of about -100° C.
 4. Process accordingto claim 1, wherein the hydroxyl group of the alcohol is protected inaccordance with known methods and the protected alcohol is then oxidizedby means of an oxidizing agent selected from sodium periodate in thepresence of a catalytic amount of a ruthenium derivative, or potassiumpermanganate.
 5. Process for the preparation of baccatin III and10-deacetylbaccatin III derivatives of formula ##STR11## in which R is aphenyl radical or a tert-butoxy radical and R₂ is a hydrogen atom or anacetyl radical, comprising treating benzylamine with an agent forintroducing a benzoyl or t-butoxycarbonyl group, to obtain the productof formula ##STR12## which, after double anionization, is reacted withacrolein to give the alcohol of the formula ##STR13## in the form of asyn and anti mixture, from which the syn form: ##STR14## is separated,the hydroxyl group of said syn form is protected and the alcoholprotected in this way is then oxidized to give the phenylisoserinederivative of formula: ##STR15## in which R₁ is a protecting group forthe hydroxyl group, in the syn form, the enantiomers of which areoptionally separated, and which is reacted with a taxane derivative offormula ##STR16## in which R₃ is an acetyl radical or a protecting groupfor the hydroxyl group and R₄ is a protecting group for the hydroxylgroup, and then the product of formula (II) is isolated after replacingthe protecting groups R₁ and R₄ with hydrogen atoms and optionallyreplacing R₃ with a hydrogen atom.
 6. Process according to claim 2,wherein the inorganic base is sodium hydroxide or sodium bicarbonate orcarbonate.
 7. Process according to claim 2 wherein the organic base istriethylamine or 4-dimethylaminopyridine.
 8. Process according to claim3, wherein the organolithium derivative is s-butyllithium.
 9. Processaccording to claim 3, wherein the inert organic solvent istetrahydrofuran.
 10. Process according to claim 2, wherein the organicsolvent is methylene chloride.